Conventional orthogonal frequency division multiplexing (OFDM) networks and orthogonal frequency division multiple access (OFDMA) network are able to improve the reliability of the channel by spreading and/or coding data traffic and control signals over multiple subcarriers (i.e., tones). Different user devices (i.e., subscriber stations, mobile stations, etc.) are allocated different sets of subcarriers (or tones) for transmitting and receiving data and control signals. The subcarrier frequencies are orthogonal to each other, thereby minimizing interference between user devices.
OFDM techniques are particularly advantageous in multiple-input, multiple output (MIMO) wireless networks that employ multiple antennas (i.e., Smart antennas) to transmit OFDM signals to the user devices. However, in conventional MIMO techniques based on OFDM transmission, the same subband (or set of subcarriers) is allocated to a given user device from each of the transmit antennas. However, due to independent fading from each of the transmit antennas, the user device may experience different channel quality for each of the transmit antennas even for the same subband. Therefore, transmitting on the same subband from each antenna for each user devices results in less than optimal performance, because the subband selected for transmission may not be good on each of the transmit antennas.
Therefore, there is a need in the art for improved apparatuses and methods for transmitting OFDM signals from a multi-antenna OFDM transmitter to an OFDM receiver. In particular, there is a need for a multi-antenna OFDM base station that transmits to multiple user devices (e.g., subscriber stations) without suffering the performance degradation associated with independent fading of the same subband from different antennas.